Add IsAlly check to MobileUnit OnStartBuild

[clyfordv]

fixes #2725

Description of the proposed changes

Prevent mobile units (engineers) from building enemy buildings. This typically happens when two engineers of the same faction are given a build order to the same resource spot.

Testing done on the proposed changes

Spawn two engineers of a given faction, give them an order to build a mex/hydro on the same spot. Previously whichever engineer arrived second would assist with constructing the enemy structure; now they clear their command queue instead.

[Garanas]

This usually doesn't that often, but if it does - would it be natural to cancel the current command queue? It means your engineer would effectively be idling.

I think this is great as a first version to fixing the problem. But what if we'd extend the copy orders feature to allow you to skip the first N orders? That way you can copy the orders of the engineer but skip the current order and the queue remains functional. This extension would not be available through the sim callback

[MrRowey]

if your extending this like jip sudjest that fine when its ready for merge please add the snipit

[clyfordv]

Extension is in the works

[Garanas]

@clyfordv is it fine if I merge this in? You can make the improved version in a different branch, if you haven't already 😃

[clyfordv]

We're good to go, I'll add the improvements later.

[MrRowey]

We're good to go, I'll add the improvements later.

Add Changes Snipit and ill merge in